How will I know when to create an interface?

a.Process();
b.Process();
c.Process();
d.Process();

List<IProcessable> list;

foreach(IProcessable p in list)
    p.Process();

public void RunThenWalk(Monkey m) {
    m.Run();
    m.Walk();
}

public void RunThenWalk(Dog d) {
    d.Run();
    d.Walk();
}

public void RunThenWalk(IAnimal a) {
    a.Run();
    a.Walk();
}

if(myObject is Boat) ((Boat)myObject).Drive()
else if (myObject is Car) ((Car)myObject).Drive()

((IDrivable)myObject).Drive()

myBusinessObject.Save(controlA.Data);
myBusinessObject.Save(controlB.Data);
myBusinessObject.Save(controlC.Data);

foreach(Control c in Controls)
{
  ISaveable s = c as ISaveable;

  if( s != null )
      s.SaveToBusinessObject(myBusinessObject);
}

interface ILogger
{
    void Log();
}
class FileLogger : ILogger
{
    public void Log() { }
}
class DataBaseLogger : ILogger
{
    public void Log() { }
}
public class MySpecialLogger : SpecialLoggerBase, ILogger
{
    public void Log() { }
}

public interface IUser()
{

void AddUser(string name ,string fname);

}

public class  BussinessLayer: IUser,IOrder,IOrderItem

{

public void AddUser(string name ,string fname)
{

//do stuffs here.
}

//All methods from all interface  must be implmented .

}

IUser user = new (IUser)BussinessLayer();

// It will load  all methods into memory with are declared in IUser interface.

user.AddUser();