When program execution is paused at a routine call statement, clicking
  on the Step push button typically executes the called routine in one
  step (depending on the coding style used), and the debugger suspends
  execution at the next source line in the calling routine (assuming no
  breakpoint was set within the called routine).  This lets you step
  through the code quickly without having to trace execution through any
  called routines (some of which might be system or library routines).
  This is called stepping over called routines.

  To step into a called routine so that you can execute it one line at a
  time:

  1.  Suspend execution at the routine call statement, for example, by
      setting a breakpoint (see the Suspending Execution with Breakpoints
      topic) and then clicking on the Go push button on the push-button
      view of the main window.

  2.  When execution is paused at the call statement, click on the
      Step-In push button on the push-button view of the main window.
      This moves execution just past the start of the called routine.

  Once execution is within the called routine, use the Step push button
  to execute the routine line by line.

  Clicking on the Step-In push button when execution is not paused at a
  routine call statement is the same as clicking on the Step push button.