A method and apparatus for debugging software for the purpose of modification of the target program's behavior and/or collection of data pertinent to a target program's execution. New user actions are compiled and converted into a dynamically linkable module. The existing program is run under the control of a dynamic action linker. The dynamic action linker modifies the existing program by inserting the new actions in the memory image. The insertion is accomplished by automatically recognizing and modifying object code sequences in the existing program to call the new actions. Once the modification phase has finished modifying the existing program's memory image the new program is run without additional interruption, the new actions acting as if they were present in the original source code for the program.