Will a 195/60r15 tire fit on a 2008 Nissan Altima? When I looked it up it said 215/60r15. Are these tires interchangable?
Yes, most likely it will fit. 195/60r15 is smaller (narrower and less tall) than 215/60r15, but has the same rim size, so not even brake clearance should be an issue.
However, the diameter of your wheel would reduce from 639 mm to 615 mm, by 3.9%. This means your wheel rotates 3.9% faster at the same speed, meaning the speedometer shows 3.9% faster than it used to show. If your speedometer error used to be that speedometer shows 5% over the true speed, now it is 8.9% over the true speed.
Your ground clearance also would reduce by half of the diameter difference, i.e. 12 mm. On today's cars with low ground clearance, this is a modest decrease. If your ground clearance used to be 134 mm, now it will be only 122 mm.
My choice would be to stick with the recommended tire size.
Edit: if you manage to find 195/65r15, it has only 0.71% reduced wheel diameter, and would reduce ground clearance by only 2.25 mm. I'd say this is pretty insignificant.