Object work

Object work is miming the objects or props used in the environment in which the scene will take place.  This is important as improv is generally performed without physical props -- A couple of chairs and a door on stage is generally the exception.  
Object work makes the scene a lot more believable (and interesting).  It's always a good idea to begin a scene with object work of some activity before you begin talking.  For example, one can pretend to hold a shovel and start digging.  Or one can pretend to hold a bowling ball and start bowling.  

3 ways to support the object work that your partner may have initiated in a scene (for example: a scene with a boxer who is training by punching a punching bag).

1. Exact replica.  Do the exact same activity with the same physical motions (and with the same emotions).  If your partner is boxer who is punching a punching bag with intensity.  You could pretend to also be working out nearby by punching another punching bag nearby with the same intensity.
2. Near replica.  Do the exact same activity but in a contrasting manner.  For example, if your partner looks like an amateur boxer punching a punching bag.  You could decide to look like an expert professional boxer by acting very confident and having very agile motions.  Or you could take another route and look like a very weak beginner, who can barely even punch the punching bag forward.
3. Supporting activity.  You could be stretching, folding towels, or doing some other activity that would be consistent with an environment where one would see a boxer practicing.

Other notes on Object Work:

• Object work should be done not only to initiate a scene, but throughout a scene.  So in between each time you bowl, you could grab your improv beer and start sipping that before your next roll.  Otherwise, it would look unrealistic to constantly bowl every 10 seconds throughout a scene.
• The object work should generally not be the focus of the scene.  If two people are digging a hole, it's better to discuss other things that are going on in the lives of the two people.  Two long time co-workers probably wouldn't be talking about the mundane details of digging a hole.
• If the object work is unclear to both partners, it's a good idea for one partner to clarify the situation.  This can be done by briefly mentioning something that will make it absolutely clear what activity they are doing.  (E.g. If your partner looks confused when you've been trying to pump air into bicycle tires for the past 15 seconds, you could mention to your partner, "You want to make sure that you pump these tires throughly.  We can't afford to have any more angry customers at this shop.")
• You (and your partner's) demeanor is also important!  It's been said that communication is mostly non-verbal.  So pay attention to how your partner is performing the object work (does he look upbeat, depressed, relaxed) and respond to that.