function custom_form_alter_form_alter(&$form, &$form_state, $form_id) {
    if ($form_id == 'form_id') {
                if(isset($_SESSION['sesstionvariable1'])){
                            if($_SESSION['sesstionvariable1'] == 'var1'){
                            $form['field_role']['und']['#default_value'] = '1';
                                }
                        elseif($_SESSION[sesstionvariable2'] == 'var2'){
                            $form['field_role']['und']['#default_value'] = '2';
                                }
    }
}