I prefer the "in media res" opening for this. The party is already together on a wagon, in the woods hiking, part of the militia... and the action set piece happens that they were brought together for prior to the game starting. So if they were hired as guards (for example) to protect a wagon shipping stuff to a town, they are already on the road and the attack happens that they were specifically hired to be defense for. This forces everyone at the table to immediately jump into action and we all see how these PCs (and their players) react... and everyone makes character choices as part of dealing with the situation that the other players learn about.
Then once the initial combat ends... I might at that point do the "flashback" to them at the tavern/job board or wherever it was that brought these PCs together in the first place and we can do a bit of RP that start having them make connections and start to come together. And now that they've had at least one fight under their belts to help define who they are, the players have an easier time of playing their PCs in the downtime and establish the relationships a little more. After which we then jump back to the present and pick up after the fight.
Now yes, some people might complain that I am "railroading" them into what their initial job was / why they came together as a party and then "railroaded" them into an initial fight... but I don't give a rat's ass about that. This is one of those times when I think "railroading" is a perfectly reasonable way to play and why I roll my eyes whenever someone says "Railroading is always bad". Nope. Sometimes a moment of railroading is perfectly fine... and using it to get the team together so the game can start is one of those times in my opinion.