How did theropods stalk prey in open areas?

Most representations I've seen of possible theropod hunting behavior (think Prehistoric Planet) shows them taking advantage of trees or very uneven terrain to hide their profile.

Current members of Carnivora, especially felines, crouch down very low while hunting in grassland. I know grasslands didn't exist until the Oligocene, but open and flat areas certainly did.

So, how did theropds do it?