There are hundreds of superstitions surrounding the theater, so it's no surprise that there are rituals involving flowers as well. It's considered very bad luck to receive flowers at the stage door before a performance, but very good luck to get them after the final curtain call. It's another superstition in some areas that real flowers should never be onstage. A few companies traditionally present the director with a bouquet after the final performance--a bouquet that has been stolen from a graveyard.
Opening night is a time-honored occasion to give flowers to an actor--it's a way to wish him well or to congratulate him on a job well done. Opening nights are usually hectic and nerve-wracking for performers, so if you want your flowers to arrive before the play it's probably best to have the florist deliver them directly to the theater. (Be sure to include a vase as part of the delivery if you want the flowers to last through the night.) If the actor in question doesn't have a dressing room, it may be best to deliver the flowers yourself after the play so he doesn't have to find a place to keep the bouquet during the performance.
Some theaters strongly prefer that flowers be delivered backstage before the performance, and some prohibit the practice due to lack of room. Call the box office to find out the protocol for your theater--and if you bring the flowers yourself, don't be surprised if you aren't allowed to present them directly to the recipient. Some theaters don't let anyone but the cast and crew backstage before a performance.
In some places there's a lovely custom of audience members going backstage after a performance to congratulate the performers and if this happens at your theater, it's a good time to deliver your flowers. There are often rules against approaching the stage during curtain calls (or the orchestra pit may make it inaccessible), but you can meet your friend in her dressing room or the lobby after the play and give her the bouquet at that time. If you do bring flowers to the theater to give after the play, consider where you will keep them during the performance; it's hazardous to lay them on the aisle floor and if they're strongly scented they may disturb the people around you.
In some venues (particularly outdoor theaters) it is customary to throw flowers onstage after a performance, especially at a curtain call. If you're attending a play in a theater that carries on this tradition (call the ticket office to check, if you aren't sure), be smart about it if you decide to join in. Throw single stems of lightweight, tidy flowers such as roses or daisies, not whole bouquets. Be sure any thorns are removed from the stems and toss the blossoms gently underhand rather than pitching them as hard as you can into the actors themselves. Aim for a gentle rain of flowers at the front of the stage, not a mass of greenery under the actors' feet; you don't want to cause anyone to really "break a leg."