I'm going to play a campaign of D&D using the Starter Set. I will be the DM, and it is going to be a 1 on 1 campaign1.
The 3rd approach of this answer appeals to me as a way to do this without too much effort, while it still leaves the chance for 2 first timers (as we both are) to concentrate on roleplaying one single character each. I want to present the roleplaying aspect to my companion. I chose this approach since I feel that it will be a less intense experience if there is no PC to interact with
I'm planning to play a character so that my friend can play one character as well.
I'm planning to decrease the monster count so that the XP threshold comes closest to the equal dificulty-class for just two players.
For bosses I bought the Monster Manual, so I can look for a boss-like monster that still fits in the lore with equally fitting difficulty.
The hard part, and my problem:
I know myself well enough to know that when I will be DM'ing a "story" that I already know, while I'm playing a character that is part of it, that as long the character itself has a motivation to get ahead, I would play it in an impatient, pushing way. I am not good enough at "winging" the behavior of the character for every situation, so I need a design with limits and restrictions on the GMPC. It's all well and good to have a plan that will respond in a certain way to a situation, but this won't help when things come up unexpectedly.
Where I am now in GMPC development:
The PC is going to be the pre-created rogue of the Starter Set story and the GMPC is going to be the Mage with his book of life story as named bond.
I need to characterize a trait, behavior or background for the GMPC that it has no ambition to push the story ahead. (The GMPC must leave the unfolding of the story in my companion's hands -- my companion will be making decisions and choices.)
As I tried out ideas for this character concept, I discovered a flaw:
When I asked myself "What is his motivation to even participate in this adventure?" there was no feasible answer to it.
How do I design a GMPC so that his trait, behavior or background -- when role played correctly -- prevents him from initiating decisions, or otherwise participation in decision-making,2 while still having a valid motivation for the success of this two person party?
1 Among the cons of a starter pack, as I assume most know, is its difficult to play in this given set up.
2 Should the character be forced by another PC into making a decision, it is acceptable, since it is an exceptional case not caused by the GMPC's player himself.