The JIT always runs - there's no interpreter, so if your need to run a method the first thing the VM does is to JIT compile it.

This has some disadvantages - you have to wait for the JIT to run and can't keep running in the interpreter while it compiles, and some advantages - it's simpler.

No, when .NET came out computers had gotten fast enough that the JIT pauses around startup were manageable, so they didn't use interpretation as a stopgap like the JVM did (which was created much earlier).