Quick demo of how optional parameters work with interfaces.

OptionParametersOnInterface.cs

namespace OptionalParametersOnInterface
{
    using System;

    public interface IFoo
    {
        void DoStuff(bool someBit = true);
    }

    public class Foo : IFoo
    {
        public void DoStuff(bool someBit = false)
        {
            Console.WriteLine(someBit);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Foo foo = new Foo();
            foo.DoStuff();
            // Outputs False

            IFoo ifoo = foo;
            ifoo.DoStuff();
            // Outputs True
        }
    }
}