CS5 Mixing frame rates in a 23.976 feature film project

Yes, you'll want to. Interpret footage is basically saying "take these frames and play them back at this speed"; it's not adding or subtracting frames. This will create a natural slow motion without using time remapping/speed, but you'll need to readjust your in and out points as they will shift after re-interpretation.

Angelo Lorenzo
Fallen Empire - Digital Production Services

You can use a speed adjustment and then right click the clip in the timeline and make sure frame blending is turned on.

For a more high quality process, you could take the file, bring it into After Effects, interpret it as 23.976 to get some slow motion out of it, and then use the timewarp effect to bring you the rest of the way. Timewarp is based off of The Foundry's Kronos effect and you can do things like interpolate extra motion blur and so on. Alternately, you could purchase the plugin Twixtor which does nearly the same thing but I find it to be more convincing for some footage.

Angelo Lorenzo
Fallen Empire - Digital Production Services