You can change the interpolation type in the Timeline panel, using the Settings button. I have it set to Linear, because it is what I use the most.
If you want to automate physics, you might want to look into what user macton has done in the scripting forum.
It seems his scripts aren't hosted anymore, so I put the crashcore scripts on my .Mac public page for now: You install them in the menu scripts folder of your Anime Studio Pro folder (see screenshot). I have no idea if you can use the scripts with the regular version of Anime Studio (probably not). You activate the scripts through the Scripts menu in Anime Studio.

The most useful scripts are IMHO:
- CC: Copy Selected Curve
- CC: Paste Curve To Points Position
User macton uses step interpolation in his scripts, so you full control over the position of your vector object. It makes things so much easier.
BTW This is the modified version, in which I removed a bug that occurred if you had any actions going on.