After Effects smoke effect

1. Car crash without crashing - a 7-second shot:

• The easy way - look for a shot of the same make/model/color that has been crashed, then plan your shot accordingly so you can overlay the crashed car.
• The hard way - export a frame of your footage to Photoshop and edit the PSD so it looks like the car is crashed (estimated time 2 to 4 hours)
• The even harder way - Camera track the shot, insert a reference solid that is on the same plane as the car, export a Maxon C4D file of the shot, open up C4D lite - add a 3D model of the car and bend it so it looks crashed (estimated time 3 or 4 days)

2. Adding atmospheric smoke to a shot - a 10-second conversation with tow actors and one camera angle:

• The easy way - purchase some stock footage of smoke or generate your own smokey layer using Fractal Noise, add it to the timeline above the shot, and use the Add or Screen blend mode and opacity adjustments to get the look you want (estimated time 15 minutes)
• A little harder way - use the same technique for generating smoke as in the first easy way, rotoscope the actors and any other geometry you want to separate from the background, then layer copies of the smoke layers with the masked (rotoscoped footage) to get a more realistic effect (estimated time 2 to 10 hours)
•  The best, most time consuming and most realistic way - shoot your car shots on a greenscreen stage or tow the car on a process trailer, key out the actors, separate the shot into 3 or 4 levels, film background plates that match, and then create the composite by layering (estimated time - about a week)

In all cases, the middle option will give you very acceptable results. All techniques require fairly extensive knowledge of masking, layering, and blend modes. If the camera is moving, you'll also need to be comfortable with tracking. If you are new to AE please spend a few hours with the User Guide. There is no click a button solution to either production problem. It would also help us help you if we had some idea how much experience you have filming and working with After Effects. For either of these shots to be pulled off in any kind of believable way, you'll have to shoot them with the effects in mind. Just pointing the camera and turning it on can add months to your post-production time.