A strong safety splits their duties between crushing the run and breaking up passes. If you're interested in playing strong safety, you need to have a unique skill set in being good at pass coverage, like a cornerback, but also be able to tackle like a linebacker.
Here's your alignment, assignment, and read for strong safety:
If you're in a basic defense, you'll line up on the strong side of the formation (how to determine which side that is), typically about six or seven yards deep and a few yards outside of the end man on the line of scrimmage. This allows you to be in position to stop the run or pass.
Your assignment is to stop the run (or force it inside) if it's run, and to drop in pass coverage if it's pass. The sooner you can determine which it is, the better chance you'll have of coming up with a big play, so have your eyes locked in to get your read.
You'll key the end man on the line of scrimmage to determine the initial run/pass read. If he is charging trying to move the guy in front of him out of the way, it's most likely a run play. If he pops up or back pedals, that's a pass read.
If you're in man-to-man, find your receiver and cover. If you're in zone coverage, drop as quickly as possible into your assigned zone, then get your eyes back to the quarterback, looking for any receivers threatening your area of coverage.
If you read run towards your side, your responsibility is to either make the tackle, or force the run back to the inside so the rest of your team can help. If a back or lineman is barreling down to block you, you need to cause a collision that forces the running back to cut it back inside. If it's run away from your side. "stay at home." Don't take off to the other sideline or you might get burned by a reverse play. Your teammates on the other side of the field can handle their responsibilities. Once you're sure it's not coming back, then pursue the ball from the inside out.
Who Should Play Strong Safety?
A strong safety has to be physical. He will play a critical role in stopping the run, so he has to be able to tackle. However, teams today are passing more than ever. So, a strong safety also has to have great feet, great quickness and speed in order to cover receivers. If you get to play strong safety, you'll get to fly around all over the field making plays on both the run and the pass.