Congrats on getting past stage 1.
Well the kind of stuff i guess you should watch out for is standard question which will get you up and running. Simple but its to easy you into the interview.
1: Simple 3d maths. cross\dot\geometry\trig.
2: How would you go about debugging a shader which is buggy, procedure for finding the bug etc?.
3: Possibly asked what effects you’ve created in the past, DOF, HDR, motion blur etc.
4: Some C\C++ questions (standard).
5: Possible asked about your appreciation for development on limited platforms (i.e. working in memory restrict environments such as consoles etc).
6: Possible be given a particular problem to solve or case: explain how implement a simple rain effect\lightening effect?
7: might even have to write and debug a simple programming problem.
These are just possibilities, but these are the kind of things we’d be asking. If your being recruited into a graduate role then it probably wont get to focused in one area. But if its totally aimed at 3d programming and nothing else be prepared to answer questions about GFX techniques; culling method etc. best of luck