I have some investors that I just pay a flat interest rate while their money is being used and they fund the entire project. If they do fund the entire project, they get a deed of trust (mortgage) on the property in the first position. Other investors I don't pay any interest, but they get 1/2 of the profit. And some investors get both if they are willing to give me flexibility with their money. I.e. if they let me use their money combined with a hard money loan with no deed of trust. Investors that just get a flat interest rate (10-12%) are my preference because they don't care how the money is used (rentals or flips) and they are purely passive. I also pay the interest payments monthly using direct deposit.
It all depends on what their goal and yours is. If they just want some diversification with a consistent return, flat interest is the way to go. If they want to maximize their return, then do a profit split, usually not more than 50/50. If you just want to get one under your belt and build investor rapport, you may be willing to pay them more at first to prove the process. But once you set the expectation with them, it's hard for them to swallow getting less on future projects. I started out giving a lot to my first investors to build my business (10% interest and profit split), and then less to new investors (flat interest with no profit split).