Twitter's amazing because suddenly being on the telly is like being on stage: you get this immediate response and it's really unusual. On Sunday [when the last episode of Sherlock aired], it was extraordinary. I got hundreds of messages and 18 marriage proposals.
Obviously a big difference between non-live audience TV and theatre is the lack of audience response. Having immediate feedback on the twitter to a TV broadcast can't impact the performance in the way that it does in theatre, but perhaps does provide some of the same feeling for the actors.