At the end of the level, just before you destroy the helicopter and after all the enemies on the ground are dead, stand just outside the last door (when leaving the building stay to the right, but turn back towards the door). An enemy keeps respawning, and he comes out the same way every time. Grab him as soon as he comes out and take him back inside to interrogate, then kill him. Go back outside and wait to do it all over again. If done correctly, the helicopter will not hit you, and you can really rack up the point multiplier. Use all the different interrogations you can and alternate quick kills with knock outs, to boost the multiplier. You can get up to 450,000 points to spend on character upgrades by doing this about one hour.

If you need more points for upgrading, replay the stark towers level and take out the guard that appears every time from the doorway. If you do not get hit by the chopper, then you can interrogate and quickly kill him and repeat the process. He appears from the door behind you where you came in. Another easy way is to beat your original scores after completing all the levels.