If a base class already implements an interface, and in a derived class you wish to implement the same interface overriding only certain methods of that interface, you just reimplement the interface and set only the interface methods you wish to override.

In this example MamanDerivedBaz is derived from MamanBaz. Both implement the MamanIbaz interface. MamanDerivedBaz only implements one method of the MamanIbaz interface and uses the base class implementation of the other.

static void
maman_derived_ibaz_do_action (MamanIbaz *ibaz)
{
  MamanDerivedBaz *self = MAMAN_DERIVED_BAZ (ibaz);
  g_print ("DerivedBaz implementation of Ibaz interface Action\n");
}

static void
maman_derived_ibaz_interface_init (MamanIbazInterface *iface)
{
  /* Override the implementation of do_action */
  iface->do_action = maman_derived_ibaz_do_action;

  /*
   * We simply leave iface->do_something alone, it is already set to the
   * base class implementation.
   */
}

G_DEFINE_TYPE_WITH_CODE (MamanDerivedBaz, maman_derived_baz, MAMAN_TYPE_BAZ,
                         G_IMPLEMENT_INTERFACE (MAMAN_TYPE_IBAZ,
                                                maman_derived_ibaz_interface_init)

static void
maman_derived_baz_class_init (MamanDerivedBazClass *klass)
{

}

static void
maman_derived_baz_init (MamanDerivedBaz *self)
{

}

To access the base class interface implementation use g_type_interface_peek_parent from within an interface's default_init function.

If you wish to call the base class implementation of an interface method from an derived class where than interface method has been overridden then you can stash away the pointer returned from g_type_interface_peek_parent in a global variable.

In this example MamanDerivedBaz overides the do_action interface method. In its overridden method it calls the base class implementation of the same interface method.

static MamanIbazInterface *maman_ibaz_parent_interface = NULL;

static void
maman_derived_ibaz_do_action (MamanIbaz *ibaz)
{
  MamanDerivedBaz *self = MAMAN_DERIVED_BAZ (ibaz);
  g_print ("DerivedBaz implementation of Ibaz interface Action\n");

  /* Now we call the base implementation */
  maman_ibaz_parent_interface->do_action (ibaz);
}

static void
maman_derived_ibaz_interface_init (MamanIbazInterface *iface)
{
  maman_ibaz_parent_interface = g_type_interface_peek_parent (iface);
  iface->do_action = maman_derived_ibaz_do_action;
}

G_DEFINE_TYPE_WITH_CODE (MamanDerivedBaz, maman_derived_baz, MAMAN_TYPE_BAZ,
                         G_IMPLEMENT_INTERFACE (MAMAN_TYPE_IBAZ,
                                                maman_derived_ibaz_interface_init))

static void
maman_derived_baz_class_init (MamanDerivedBazClass *klass)
{
}

static void
maman_derived_baz_init (MamanDerivedBaz *self)
{
}