I was sitting in the Los Angeles Farmer's Market recently doing a script consulting session, when the subject of multiple casting arose. It's hardly uncommon in these days of tight theatre budgets. Certainly, casting one actor instead of two or three or even ten is easier on the pocketbook, gives that actor more to do, and is less of a hassle to coordinate from a scheduling standpoint.
No one will argue the practical advantages of multiple casting (assuming, of course, that the actor isn't running around backstage like a madman trying to deal with fifteen second costume changes). But at the same time, no one has ever come up to a playwright and said, "I love how you saved $300 a week on your casting. That was a beautiful moment."
On the other hand, sometimes multiple casting is just what the script doctor ordered. For example, in my play Ben, the title character, a teenage street kid, strikes up a friendship with Baxter, a local restaurant owner who happens to be gay. Baxter is a great guy, but I deliberately doubled him with Dryer, a social worker who molested Ben when he was younger. The intention, by doubling these two characters, is to allow Ben to see in Baxter that same possibility, thus increasing the dramatic tension. No one ever literally draws the connection; the doubling does that.
In another play, my free adaptation of War of the Buttons, one actor plays all of the adult characters, to make the point that, at least to the kids, the adults are all similarly disconnected from them. The one actor becomes something of an Everyman or an Anyman, removed from the world of the young people who populate the play.
And don't be afraid to let the theatricality of multiple casting show. That might mean having the actor switch from one character to another in front of us. (Remember, it doesn't have to be a full out costume change - something minor, like the addition of one piece of clothing, might be enough.) The trick is to find a way to connect that switch with what is going on in the play, rather than simply let it be arbitrary.
What common thread can you find between the multiply cast characters? What does the choice of multiple casting do to increase the tension, to inform us about the play and its characters (all of them, not just the multiples)? When you can answer these questions, you're on your way to using multiple casting as more than just an efficiency measure.