For our shows often we have students with two or sometimes more roles (once we had a great little actress play 5 roles!!).

 Our biggest problem never seemed to be getting the character right but getting costume changes done on time!

 I have in one show, played a commoner and her queen.  An interesting experience to be sure.  Lucky for me they were extremes.  Also in a whacked out version of the Shrew I played a scene as Katerina and was fully cast as Baptista Minoloa (who happened to be a famous Soap Opera actress).  Both were exciting experiences.  As I said though, the main problem was changing from Baptista's mini dress to Kate's in a matter of a few minutes!

I don't think there is any copyright violation in this regard.  Hard to say. But why would there be if there is??  Can't really think of one - especially if for instance, an emergency arose and one person had to drop out last minute or something - only to be able to fill it with someone already familiar with what is going on onstage - that's either the director or another actor who might be filling in a smaller role someplace else in the show.  So actors CAN and DO double all the time...in shows where you really don't have a way to accommodate the masses.

There is a show I directed a number of years ago - called THE DINING ROOM.  FUN show to do and much like TristanRobins show with 60 characters - this one had 6 or 7 actors doing anywhere from 4 - 6 characters each or thereabout (can't remember too long ago).  But the KEY I think - from the standpoint as a director first is that you cast people who you know can handle those scene to scene transitions and shifts of character.  Now if YOU as a director can't give them what they need to be able to do that should they not readily possess that capability, then perhaps it's a bad choice to try to direct something of that specific nature.

As an actor, I think what it boils down to is nothing more than analysis - sheer HOMEWORK working toward finding those things which define clearly each character that has to be portrayed. There is one scene where 5 or 6 of the characters are children at a dinner table at a birthday party.  And in the next scene they're adults with one of them as an old lady.  But you just have to find the things about each individual character that clearly defines who they are and then work toward acting out those characteristics.  I'll tell you - I wouldn't want to do it as an actor.  I WAS able to do it as a director becasue first, I had GREAT actors.  But secondly as a director it was like directing 30 - 40 some odd DIFFERENT character actors. All I could convey is what I saw NOT out of the 6 or 7 actors I used, but what I needed to see from AN ACTOR PLAYING A INDIVIDUAL ROLE!!!  As I recall, I think I even did some shifting around of character types from the script, utilizing the strengths that each actor possessed to be able to handle the "character type" I felt they could best portray.  Now as in TristanRobins case - with only 4 actors playing 60 roles - let's see the math = 15 roles apiece more or less depending upon what the script called for.  THAT is tougher still.

Honestly I think it's just a matter of strength of the actor and the ability to consciously SHIFT gears without flinching, and NOT carrying one character over into another UNLESS there are SUPPOSED to be similarities.  Takes a lot of thought and a lot of homework.  FURTHER, it takes an audience a little time to "get it" or to "catch on" but once they do - and that has to happen quickly in the beginning of the show - but once they do, then it becomes much easier for an actor to "put it out there" and have it be accepted - EVEN if for some reason there is some occasional bleed over of one character in one scene to another character in another scene.  Audiences - by that time (we hope) have caught on and are willing to take the ride with you regardless.  BUT MAN it IS HARD to do.  It is a great exercise though for an actor.

Sounds like fun.  Be sure to HAVE some fun.  Drastic changes can make it a lot easier...subtle changes are harder to pull off. Good luck.

Some shows specify in the cast breakdown "with doubling" or "flexible casting".... language to that effect.

The example of "The Dining Room" was brought up as one such show.  It is traditionally performed with several actors taking on multiple roles.  So is "The Laramie Project".... and "How I Learned to Drive". 

From Dramatists website, here is the link to "The Laramie Project": http://dramatists.com/cgi-bin/db/single.asp?key=2955 - http://dramatists.com/cgi-bin/db/single.asp?key=2955

And, "How I Learned to Drive": http://dramatists.com/cgi-bin/db/single.asp?key=2683 - http://dramatists.com/cgi-bin/db/single.asp?key=2683

And "The Dining Room": http://dramatists.com/cgi-bin/db/single.asp?key=1279 - http://dramatists.com/cgi-bin/db/single.asp?key=1279

It's pretty clear for these shows that flexible casting/doubling is okay (in fact, encouraged).

Obviously, plays that are in the public domain (such as Shakespeare and Wilde) can be cast any which way, as no copywrights are in effect.

With regard to shows which don't necessarily specify if doubling can be used, if you're not sure, then perhaps your question can best be answered by inquiring at the publishing house.