Original Cast Precedent

Doesn't it sometimes seem like, when someone does Jesus Christ Superstar, Judas is always black?

This may not technically be true--there may indeed be as many non-black actors cast in the role as black ones--the point is that the impression is there. Most people who know the show, or at least the Broadway production with Ben Vereen and film with Carl Anderson, when they think of the character, think of him as black. This phenomenon is also why most people who think of the musical Rent will imagine Mark with short blond hair and glasses. This can also refer to casting conventions, such as how certain shows are always subject to (sometimes ridiculous) Dawson Casting whereas others are less so.

Whether or not this refers only to things not listed in the script or things that may be listed in the script but are irrelevant to the characters and story is up for debate.

Notable original cast precedents include:

 * In Into the Woods, the actor who plays Cinderella's Prince doubles as the Wolf, the the Narrator as the Mysterious Man, and Cinderella's Mother as Granny and the Giant. This is not a script-based necessity, merely a tradition established by the original Broadway production.
 * It also makes thematic sense, especially between the Prince and Wolf and the Narrator and Mysterious Man.
 * Nathan Lane codified the performance of Max Bialystock, so much so that in a brief Fourth Wall-breaking moment in "Betrayed," Max will pull out a Playbill and remark "He's good, but he's no Lane!".
 * Inverted with Pippin. The Leading Player was originally played on Broadway by Ben Vereen. However, there really is no typical image of the Leading Player, and 'he' is as often a 'she' as not, and of all different ethnicities, body types, and ages (as long as he/she is older than Pippin). Pretty much the only stereotype of this role is that it's hardly ever played by a white male.
 * Another example is that the original actor for Pippin had absolutely no luck with the costume department in regards to shoes. He could never find a pair of shoes that were comfortable, so one night, fed up, he decided to do the whole show barefoot. It wasn't easy. In his dressing room after the curtain call, Bob Fosse (the choreographer) came in. The actor prepared himself for a grovelling apology, but instead Fosse gushed, "I love it! Barefoot! Gives you that innocence." Since then, Pippin is always barefoot.
 * This is probably where the utterly ridiculous tradition of casting a middle-aged man as Hamlet came from: Richard Burbage, who originated most of the titular roles in Shakespeare's tragedies, was in his late thirties or forties when he originated this role. This is mostly justified by the equally ridiculous throwaway line in the uncut text about Hamlet being thirty, or possibly because people feel someone in their late teens wouldn't have the acting range to pull off Hamlet, but even so, some of the actors who have played him have been most definitely not thirty. It long ago reached the point where "I played Hamlet!" is fictional shorthand for "I've done more theater shows than you can count!"
 * In Sweeney Todd: The Demon Barber of Fleet Street, Mrs. Lovett is almost always cast a little on the plump side or blowsy, like Angela Lansbury as opposed to the equally (perhaps more) likely possibility of her being thin and bony. Christine Baranski is one of the few skinny Mrs. Lovetts.
 * There also seems to have been a shift based on the Revival's take on her. Angela Lansbury made her rather grandmotherly, but Patti Lupone's version is younger and more in the way of a Perky Goth, a presentation which also applies to the film version (although most Tim Burton characters have Looks Like Cesare going on anyway).
 * Utterson in Jekyll and Hyde tends to be played by a black actor, despite no particular reason for this.
 * Higgins' songs in My Fair Lady were meant to be sung, not spoken, but many portrayers follow the precedent of original non-singer Rex Harrison.
 * And has there ever been a production of Camelot with an Arthur who could sing? The original Arthur was Richard Burton, and Richard Harris played the role in the movie and later on stage.
 * Indeed, it's a tradition for any big budget version of Camelot to have Arthur played by a respectable movie star with little or no singing experience (Burton, Harris, Laurence Harvey, Gabriel Byrne, Michael York) or someone who made their theatre career singing that way (Jeremy Irons.) A rare exception was Robert Goulet -- the original Broadway cast's Lancelot -- playing Arthur in a touring production ca. 1998-1999.
 * The role of Peter Pan has almost always been played by a woman.
 * Similarly, the children's father and Captain Hook are generally played by the same person. (Though Barrie originally wanted Mrs. Darling to be the actress to play Captain Hook... make of that what you will.) Also, in The Musical at least, the role of Eliza, the maid, is typically doubled with that of Tiger Lily.
 * The Wiz (an adaptation of The Wonderful Wizard of Oz) was originally staged with an all-black cast, and the movie version and many subsequent productions have followed suit. However, perhaps since race is not an in-story issue, it's not uncommon to see colorblind stagings, especially on the amateur level.
 * Almost every play of Alice in Wonderland seems to include an Alice who is blonde and blue-eyed, despite the fact that the real-life Alice Liddell was dark-haired. (The original book's illustrations depicted the character as blonde.)
 * And for Alice In Wonderland in general, the main time when you do see a dark-haired Alice is when the story's been Grimmified. (Though this isn't done in every dark retelling. Just a lot of them.) Perhaps this is because it makes her look more like a goth.
 * Revivals of The Cradle Will Rock often do without costumes, scenery or orchestra; the original production (which was directed by Orson Welles, by the way) did this out of necessity when the actors were locked out of the theatre it was supposed to open in.
 * The voice of the Man-Eating Plant in Little Shop of Horrors is usually that of a black singer, though his (real) face is unseen -- or rather, is seen only as that of a Skid Row bum, which he doubles as by precedent.
 * Audrey is nearly always blonde, or given a blonde wig.
 * Zero Mostel had a huge influence on future portrayals of the characters he played. Most notably Tevye from Fiddler On the Roof--a dirt poor Russian Jew shouldn't logically be a very fat man.
 * Guys and Dolls--Sky Masterson is the traditional romantic lead while Nathan Detroit is the more comic role. Though Nathan Detroit was originally played by the tone-deaf Sam Levene, thanks to Frank Sinatra's portrayal in the movie, Nathan Detroit is often played by the better singer.
 * Broken with the Red Bird in Cirque Du Soleil's Mystere -- it was always a male role until the artistic directors of the show realized a certain female member of the company was better suited to the character's personality. It required a new costume design, since the original was designed as a Walking Shirtless Scene, but worked out so well that the role can now be filled with a performer of either gender.
 * In Phantom of the Opera Christine is almost always a brunette. It should be noted that RUG tends to keep a very close rein on character designs in their productions. There is however a production of note where Christine is in fact blonde.
 * Everyone remembers Joanne of Rent as black, because she was in the original Broadway cast. When the time came for the film, Fredi Walker, who felt herself too old to play a lawyer just out of law school, made one request - that Joanne remain black - and so Tracie Thom got the role.
 * In fact, most of the characters in Rent appear to be closely associated with the original performers. Roger tends to be a blonde, Maureen almost always appears with curly brown hair, the ethnicities of all characters tend to stay pretty much the same and, of course, the before-mentioned blond, bespectacled Mark (especially strange given that he's Jewish). This was probably exacerbated by the film version casting the majority of the original cast.
 * Averted by the 2007 Australian production, which cast the Italian Anthony Callea as Mark.
 * The only notable aversion to this in Rent is Angel. the role was originated by Wilson Jermaine Heredia, who's Dominican; later Angels have included Jose Llana (Filipino) and Telly Leung (Chinese), and the original understudy, Darius De Haas, is black.
 * All of this aside, there's also the matter of voices (inflections, center of tone, and general voice qualities). For example, although Ko-Ko in The Mikado is generally kind (and debatedly The Woobie) due to the casting of a certain John Reed, almost every portrayal you see has the same character voice. Which wrecks havok on those preparing to audition for the part, who have to choose whether to read the part in their own voice or the precedented one. Similarly, actors have imitated Victor Moore's character voice as Throttlebottom in Of Thee I Sing and Moonface in Anything Goes.
 * Hairspray had Divine playing Tracy's mom Edna, and the stage shows and later movie continue the man-as-woman tradition.
 * In Les Misérables, the majority of Eponines have had dark hair, despite the fact that the novel describes her as a blonde. This is probably set by original Eponine, Frances Ruffelle and reinforced by notable 10th Anniversary Concert Cast Eponine, Lea Salonga, and 25th Anniversary, Samantha Barks. In fact, Megan Lawrence, a blonde Eponine on Broadway, commented once to an interviewer that she never thought she'd get to play Eponine because she was a blonde.
 * Another Les Miserables example is that Cosette, another blonde in the book, always is given a brown wig, regardless of the hair colour of the actress. Katie Hall is a notable aversion. She played Cosette in the 25th Anniversary Concert. However, she too wore a dark wig when she played the role at the Queen's Theatre.
 * Rebecca Caine, who created the role of Cosette in 1985, has said that only the original cast will ever know just how much they contributed to the show. The harmonies between Eponine and Marius were created during rehearsals by Frances Ruffelle and Michael Ball, the vocal scale in "I Dreamed a Dream" was thought up by Patti LuPone, and the high grace notes in Cosette's vocals were due to Rebecca - and that's just for starters. This is probably true not just for Les Mis, but for most other theater productions of all kinds.
 * Helene in Sweet Charity is traditionally played by a black actress, like Thelma Oliver in the original Broadway cast and Paula Kelly in the 1969 film.
 * In productions of the Rocky Horror Show, Eddie and Dr Scott are frequently played by the same actor. The film, which casts two different actors in the roles, is a rare aversion of this.
 * In Das Rheingold, at the point where Wagner's text merely reads "as if seized by a great thought," Wotan usually holds up a sword left over from Fafner's hoard and points it towards the castle, in accordance with the "sword" Leitmotif which makes its first appearance here. This practice was approved by Wagner for the inaugural Bayreuth production of 1876 (though Das Rheingold had its premiere seven years earlier).
 * Bram Stoker never describes Dracula as wearing a cape. The image came from an early stage production, which included the cape to facilitate the special effects - the actor playing Dracula would close the cape around him when he was disappearing into a hidden trapdoor, making it look like he had vanished into thin air. It got carried over into the film, and since then, the cape has become associated not only with Dracula, but with vampires in general.