Brig to Interlaken

I shall be reaching Brig from Venice , i need to reach Interlaken, do i need to reserve my seats upto interlaken, I have a swiss pass

There are no reservations possible from Brig to Interlaken.
There are 2 ways to go;
The scenic old route up the Lotschberg and through Kandersteg. In Spiez you take the connecting train to Interlaken.
The fast route through the tunnel which saves about an hours time to Spiez then the connecting train to Interlaken.
If you have the time take the scenic route, it is well worth it
Dick

Thanks, we shall work a way around it