From western Switzerland to the gateway for the Bernese-Oberland by train

The fastest option for a Geneva to Interlaken by train is to make only 1 x change of train at Bern, but is not available in every hour when setting off from Genève.

Also because there are 1 or 2 x trains per hour from Bern to Interlaken; and the gap between these trains can vary from 30 mins to 90 mins, some of the connections between trains at Bern can be longer than the usual 8 - 10mins.
And when that is the case the SBB journey planner usulaly suggests an additional connection at Spiez, in order to enable a traveller to reach Interlaken faster.
It also accounts for the variations in the end-to-end journey times which can be seen when looking up the route.

You will also need to take this journey option, if you will be travelling with a dog or a bicycle.

Making the transfer in Bern station

The gleis (platforms / tracks) in Bern station are connected by both a passage beneath the railway tracks and a bridge over them.
Making the connection is step-free as the passage way is accessed by slopes and the bridge is equipped with both escalators and elevators.
On arrival in Bern the passage way will be by the middle of the train, but the bridge will be towards the rear.

Online bookings open: up to six months ahead of the travel date

Limited numbers of Supersaver tickets will be made available on most departures, you may need to search through the connections to find them using the ‘earlier’ and ‘later’ buttons.

In some hours the only option is to make two changes of train at both Bern and Spiez, but those more inconvenient journeys don't tend to be any cheaper on this route.
So try and be flexible with your departure and arrival times and target the cheapest journey you can find on your travel date that only has 1 x change of train in Bern.

Using Saver Day Passes:

If you will be making a day return trip to Interlaken from Geneva, be open to the possibility that Saver Day Passes are likely to be a cheaper option than booking return tickets

Seat reservations:

Seat reservations are optional on Swiss IC trains, so seats won't be assigned when booking online.

You can book reservations online for journeys by these IC trains, AFTER you have booked a ticket, by using the SBB (Swiss national railways) seat reservation service - OR request a reservation when booking at a station ticket office.

Note that seat reservations won't be an option if you take the IR service to Bern and/or the combination of trains on from Bern to Interlaken, which require a connection between trains in Speiz.

Travelling with a rail pass:

Rail pass users can jump on any train, without having to make reservations.

Taking the more scenic 'Golden Pass' route from Geneva to Interlaken has become much easier since the launch of the Golden Pass Express service.
This journey option now involves only making one connection in Montreux station, which in this direction is typically as simple as walking across the platform from one train to the other.

There are four trains per day which are direct from Montreux to Interlaken and the connecting trains typically depart from the main station in Genève / Geneva at 06:09; 08:10; 11:10 and 13:10.
The time between trains is Montreux is around 25 minutes.

The alternative to taking the Golden Pass Express

The Golden Pass Express is operated by MOB and it takes a route through Zweisimmen and Spiez.
However, MOB also operates hourly trains which travel from Montreux to Zweisimmen, which have easy connections there with regular trains on to Spiez.
Another easily timed connection in Spiez typically result in these Montruex to Interlaken journeys with two connections, being only 20 - 25 mins slower than taking the Golden Pass Express.
Some of the trains which operate between Montreux and Zweisimmen are more special than the norm, as MOB operates its Belle-Epoque trains, with retro styling, and its Panoramic trains, with larger windows, over this route

If you will be travelling with a rail pass, this route with connections is cheaper than taking the Golden Pass Express because you can hop on and off any train.

Because the Option 1 route from Geneva to Interlaken is so much faster, the SBB websites assumes that anyone travelling between the two locations will want to take it.
So when looking up the journey to book tickets, it doesn't off this route with a connection in Montreux as an end-to-end journey.
Though the journeys from Geneva to Montreux and Montreux to Interlaken can be looked up separately - the Golden Pass Express typically departs from Montreux at 07:34; 09:34,; 12:34 and 14:34.

Booking tickets on SBB won't give access to making a reservation on the Golden Pass Express, which is highly recommended as all of the window seats on each departure can be booked from weeks ahead.
The reservations can be booked separately on the Golden Pass Express website - and rail pass users will need to reserve.
The steps which need to be taken are explained on the Train Guide for the Golden Pass Express.