Fear of standing up for what is right is the fertilizer of tyranny. If people only stand up for what is easy or safe, evil triumphs every time.
No one wakes up in the morning and says, "You know what, today I'm going to be a hero." They don't wake up planning to be a victim either. The bottom line is, what will you do instinctively when faced with doing what is right no matter what the risks? That's not saying that you act blindly, certainly you minimize risk as much as possible. What are you willing to commit to protect what is right?
At minute 1, people would reasonably expect the cops to reduce the pressure on Mr. Floyd. Minute 2- vocal demands for relief. Minute 3- intensify demands, encroach the police space enmasse, try to re-direct who their perceived threat is away from Mr. Floyd and force them to focus on a crowd rather than an individual. Minute 4- time to preserve Mr. Floyd's life is too short, use all means possible to get the cop off his neck. Minute 5- death all means possible to save his life.
Specifics on the ground would determine actions. Disturbance on the opposite side of the cop car? Move closer to cops, no physical threats- use proximity to change their physical positions? Claim to be an EMT and demand to step in to care for Mr. Floyd? I don't know, but I could not stand back and watch him die.
I've come up on car accidents where people were afraid to get blood on themselves to help someone, it washes off. Sometimes just holding a pressure point can keep someone from bleeding out, but you have to be willing to get dirty. I've hopelessly watched seriously injured people take their last breaths when there was nothing I could do. And on a few occasions, I have literally reached out and saved someone from drowning or other perils. I am no hero.
Spectators do not win the game, the players do. The players also lose, but you don't know what is possible if you don't play.
Most people will never encounter anything remotely close to this situation in a lifetime, there is no handbook on what to do, no real training you can do to prepare. But there is education that can give you tools to adapt, such as Basic First Aid (anymore it's just how to put on a bandaid without getting sued) and Advanced First Aid, CPR. Basic self defense. I'm sure you can think of other ideas, so please, by all means share them with us here. The key is only action beside documenting with a phone had any chance of changing the outcome. Who knows what one could have done.