❔ How to input a reference of a class into a function with an argument of an interface

Hero player = new Hero(20,30,10, "Hero");
Enemy goblin = new Enemy(3,3,2, "Goblin");
BattleField plains = new BattleField();
plains.StartFight(player, goblin);

class BattleField
{
  public void StartFight(IFighter fighter1, IFighter fighter2) {...}
}
class Hero : IFighter {...}
class Enemy : IFighter {...}

Argument 1: cannot convert from 'out Test.Adventure.Hero' to 'out Test.Adventure.IFighter' csharp(CS1503)
Argument 1: cannot convert from 'ref Test.Adventure.Hero' to 'ref Test.Adventure.IFighter' csharp(CS1503)