public interface IActor { ... }
public interface IHealthUser : IActor { void AddHealth(int amount); }
public interface IAction { bool Perform(IActor actor); }
public class HealthPotion : IAction
{
    public bool Perform(IActor actor)
    {
        var performedAction = false;
        var healthUser = actor as IHealthUser;
        if (healthUser != null)
        {
            healthUser.AddHealth(Amount);
            performedAction = true;
        }
        return performedAction;
    }
}

var action = Item as IAction;
if (action != null)
{
    var performedAction = action.Perform(Owner); // Where 'Owner' is IActor
    ...
}

public interface IAction<T> where T : IActor { void Perform(T actor); }

public class HealthPotion : IAction<IHealthUser>
{
    public void Perform(IHealthUser healthUser)
    {
        healthUser.AddHealth(Amount);
    }
}

// Do we have to cast and test for every possible IActor interface?
var action = Item as IAction<IHealthUser>;
var action = Item as IAction<IManaUser>;
...