Most physical action used in plays is not indicated in the script.

Yes, sometimes there are stage directions that appear in parentheses.  Some scripts have more of this than others.  These days, playwrights steer away from including any stage directions unless they are absolutely necessary to understanding what is going on in a scene.  For instance, if a character says “Here”, and then pulls a packet of unmarked bills from his jacket pocket, and the other character says “Thanks”, you’d have no reason to know that it’s a bribe and not a throat lozenge unless the playwright tells you.  But directions such as “sits” or “stands” are rare in today’s scripts.  The playwright understands that it often doesn’t matter exactly when the sitting or standing happens, and that if it does, a good actor will be able to figure it out without assistance.

But even in older scripts where stage directions are sprinkled in here and there (and sometimes these are not the playwright’s opinions but merely the stage manager’s recording of what was done in the original production, which you should not consider to be sacrosanct), there is much that isn’t included.  It’s your job as an actor to add physical movement that underscores, enhances, or adds to the fun of the play.

If you’ve ever read a Shakespeare play, you’ll know that aside from entrances and exits, there isn’t much recorded in the script in terms of movement.  Watch a good production, however, and you’ll find lots of action, especially in the comedies.  I just saw the London production of Twelfe Night with Mark Rylance and a host of exquisite actors at the Belasco Theatre in New York, and it was full of marvelous physical bits that made us all laugh.  Here’s a few photos to give you an idea of what is possible when you let your imagination loose and express yourself through more than words (you can also check out Youtube for the American Conservatory Theatre’s commedia del’arte production of The Taming of the Shrew — links in the right column — for some very physical Shakespeare!):