Can an event horizon form with the observer already inside? I am on a planet orbting a star around the center of the milky way (assume no supermassive blackhole there). By some mechanism, mass starting from the outer part of the galaxy gets pushed inward until the schwarzchild radius is reached for that mass (.25 ly according to Wikipedia). I'm under the impression that an event horizon would form from the perspective of someone looking at our galaxy from the outside. But how would it look like from my perspective at the center of the galaxy? Would I view everything as normal? I know things would start collapsing with so much concentrated mass, but would I know I was inside an event horizon?
The event horizon is the boundary in spacetime between the black hole interior and exterior. If you exist before the black hole forms and you end up at the singularity, then you necessarily passed through it at some point. You can't get inside without crossing the boundary. From the perspective of some foliation of the spacetime, the event horizon encloses no area when it first appears, then it expands outward at the speed of light or greater, eventually passing by your location.
Nothing physically happens at the event horizon; you have to know global properties of the spacetime to work out where it is. There would be no sign that you'd crossed it.
The experience from the inside would be like a big crunch on a smaller scale (so a small crunch, I guess). The singularity is in the future, not the spatial center.