getMovies.java

private void getMovies() {
        MoviesService service = retrofit.create(MoviesService.class);
        Call<List<Movie>> call = service.getMovies();
        call.enqueue(new Callback<List<Movie>>() {
            @Override
            public void onResponse(Response<List<Movie>> response, Retrofit retrofit) {
                saveMovies(response.body())
                getMovieCasts(response.body())  
            }

            @Override
            public void onFailure(Throwable t) {
                Utils.showErrorMessage(StringConstants.ERROR_MESSAGE);
            }
        });
 }

private void getMovieCasts(List<Movie> movies) {
        // Note we are doing it on new thread because
        // we need to fetch all the casts sequentially
        // and update the UI when all the casts are fetched
        new Thread(new Runnable() {
            @Override
            public void run() {
                MoviesService service = retrofit.create(MoviesService.class);
                for(Movie movie: movies){
                    Call<List<Cast>> call = service.getCasts(movie.getId());
                    List<Cast> casts = call.execute();
                    movie.setCasts(casts);
                }

                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        // Now update UI with movies. 
                        // This way we have both movies and casts together.
                        updateUI(movies);
                    }
                });
            }
        }).start();
    }