If you're in economy and you're not in a front row expect an uncomfortable ride. Their seats have a rediulous amount of recline. More than I've seen on any other airline, so much so that you're screen can't even tilt enough to cope with it. Otherwise it's pretty good service.
I'm pretty sure I was able to reserve a seat in advance but I bought my ticket direct from LH but I'm unsure how it works if you've bought an interlining ticket from another airline.