@necrodethmortem Being a 3rd driver is no guarantee of being the first stand-in. It only guarantees that you’re the emergency reserve, to be there in case a different replacement is not available (generally in short notice scenarios).
Case in point – Gutierrez was the Sauber reserve driver for 2011, if memory serves. But the stand-in for Perez (who opted out of the race because he didn’t feel well) was de la Rosa.
@Prisoner-monkeys The harsh reality is that an F1 seat, unfortunately, is not cockpit-interchangeable. The issue doesn’t lie with dimensions, it lies with the locking between the cockpit inner walls and the seat. It’s not like a chair that you just put in a room – there’s some interlocking between the curves of the seat, and the cockpit wall going on.
Shoehorning a seat into a car it wasn’t made for could work with some makeshift adjustments, but it would be a compromise. A large part of a driver’s feel comes from the base of his spine – more precisely, from the coccyx up to the third vertebra. An improper (or half-assed, for lack of a better word) seat fit will only result in a driver unable to have a good feel for the limit.
Other than that – it’s also about driver preference on positioning. See Jenson Button in 2010 to see what I mean – he was sitting half a head higher than Lewis in the seat in the first pre-season tests.
That really leaves the decision to a few drivers – should the decision be seat-limited – generally anyone who’s driven for them since 2009. Piquet, Alonso, Kubica, Petrov, Heidfeld, Senna, and JdA.