How does Ruby the String interpolation of a String object?

Is my assumption correct that Ruby does not call the to_s or to_str method when interpreting a String object during string-interpolation?

a = "Hallo"class String  def to_s"Servus"  endenddef a.to_s"Servus"endputs "Hier sagt man #{a}"

=> Hier sagt man Hallo

Is it more efficient to check if an object is a String than to simply call the to_s method?