My main group consists of me (GM) and my cousin and long time friend (Player).
I have a lot of experience running 1 on 1 campaigns as it is the type of game I run 95% of the time. The largest table I ever ran had 3 players and 1 GM.
I’ve got some personal tips I can give based on my experience.
I’ll also throw in what my 1 on 1 table is like.
Firstly, you gotta embrace that a 1 on 1 session of just 1 player and gm is a very intimate type of game. The GM only has to focus on one player and without other players, most interactions are NPC with PC.
Get comfortable with that.
GM-ing might be considerably more difficult.
For one you won’t get a break, no chance to look at your notes, sometimes players like to interact with each other and roleplay and during such times you can review your notes and write stuff as necessary, but with 1 player, that 1 player will most likely always be talking to you. So get used to multitasking.
Certain parts of play become slower without other players I found.
(Don’t run combats with more than 3 NPC’s as that means most of the combat will be the GM doing things with the player simply watching.)
Without other players to talk to, the one talking most often would be you, and the player would be listening half the time.
With only 1 player, that means you only have 1 ego you need to take care of.
Let your 1 player be the main character and always give them a chance to shine. The story, encounters, the entire game in general should all be catered to that 1 player and their character.
You can create a more novel epic story like experience that sometimes is not possible with more than 1 player because of the whole everyone should be the main character deal.
with 1 player, there is only 1 main character. 1 character who you can always involve in the story.
Instead of dungeon delving, make the game more akin to a battle show of some kind.
Uhh… Something like Power Rangers.
I found typical ways of playing, like starting off in a tavern or doing quests doesn’t work out as well when there is only 1 player.
Like I said, a television style format is more fitting.
Instead of treading through a dungeon, solving puzzles and fighting the occassional random enemy.
A main villain shows up and they challenges the player in various ways.
Combat is always a boss fight and its almost always a 1 on 1 with the player against an important character.
Generic baddies and fighting through hordes of minions and generic monsters can be fun sometimes but those 1 on 1 scenarios are the most enjoyable in this type of play.
Its quality over quantity.
Its not worth making a combat encounter against unnamed 1 off npc’s like goblins or slimes. Every enemy they fight should be important in some way and have flare.
You need to be unconventional.
This is how we play but we usually play with the player taking control of more than 1PC.
2 or 3 is a good amount of characters to let the player control.
This might not be up to everyones taste but we like playing it this way.
My cousin not only roleplays as 1PC but sometimes has pretty interesting conversations with himself simply speaking as all of his PC’s interacting with each other. Much like how a GM makes scenes involving multiple NPC’s.
He has 1 main PC which only he is allowed to play while the other PC’s I can speak up whenever I saw it was necessary. But he would always take charge of all of them in combat.
Another method of play we do, but not as often is assigning both players to be both the GM and the Player.
Both of us get a player character (maybe 2) to use and both of us simply come up with the scenarios as we go.
With both players acting as GM, there is not much room for prep and both players will have their own narrative and twists they want. Just roll with it and go with the flow, the chaos is fun when it isn’t too chaotic and twists and surprises are fun.
I guess the advice I should be giving you is relish in the intimate relationship and game this type of game provides.
Find the type of play that works for the 2 of you specifically.
The way me and my cousin play is our own intimate way of playing that works for the 2 of us specifically which we built up over a long period of time. That method of play being having multiple PC’s to play with and making the game feel kind of like a video game JRPG.