The bus driver will be annoyed if you bother him with your ticket. In Switzerland bus drivers do not check tickets of boarding passengers. You just board through any door and take a seat. It is all based on the honour system, but backed by hefty fines if you are caught without a ticket.
Anyway, I know that bus, I take it quite regularly. It is normally on time and the walk from bus to train takes 20 seconds. If the bus is late and you miss your connection you just take the next train. They go every half hour.
However if you want to take the earlier bus still, just take it. Normally your accommodation will give you a guest card, that gives you free use of that bus, so you can take an earlier bus. (So you could have just bought a ticket from Interlaken West) Otherwise just buy a ticket from the ticket machine, or the ticket office.
You will notice once here that Public Transit is easy to use, and a trivial part of everybody’s lives here.
